Personally, I would want the forensic report on the sharp force injuries to pull the sheath and the knife together. If that wasn't available (which it will be), I probably wouldn't be too doubtful about the sheath belonging to the killer - given that the sheath housed a weapon and was found underneath a dead person.
It's clearly BK's sheath, though, from my POV.
Those autopsy reports are going to be brutal. I predict that there are several strands of evidence that either exactly identify the knife (I've written about this before, but forensic MRI and forensic spectometer analysis could reveal the actual brand Ka-Bar, since it has a proprietary coating on the blade and such blades have been identified in this fashion before). But even if not, the depth of the wounds and the microscopic analysis of any marks on bones would be enough.
I do think that BK chose a manner of killing designed NOT to hit bone - but was he successful? He expected all his victims to be sleeping and not moving. Xana was moving around. I believe BK is immune to and incapable of anything like "panic" (Based on his tap=a=talk posts and general research on VS), but I do think his plan went awry when he realized Xana was still up and about. His mind may have gone blank. He writes that he suffers from derealization AND depersonalization. That explains a lot in this discussion about "how smart he was." He's (according to his own posts) neuro-atypical. There are are anomalies in his perceptual apparatus (as I believe there were in the professor I was describing in a previous post).
I do not believe he left the sheath intentionally. Kaylee was clearly still awake, the situation did not unfold as he expected/planned. I believe he was wearing something like a black coverall with those big pockets and had the sheathed knife in that pocket (he probably, while kneeling on the bed to kill Maddie, thought he slid the sheath back into that pocket, but in the haste of the moment, managed to drop it, may not have realized it). If the theory that he was after just one woman is true, then the presence of another woman (who appears to have awakened and heard him) was startling.
He wanted to walk as quietly as possible (so, no running). He had timed it in his mind, perhaps not planning to kill four people (but willing to).
He's so meticulous about packaging his trash, cleaning his apartment and car, etc., I can't believe he would intentionally leave a sheath that almost certainly had his DNA on it. However, he had not taken a grad course in forensic anthropology (which studies use points on items and also studies the genes and their various methods of extraction and reconstruction). DeSales had one master's level course in that subject, but we don't know if he took it - he was not on that particular track according to what was announced at graduation. One could easily take 9 courses in related topics (including the study of metal objects as a forensic category).
IMO.
my.clevelandclinic.org
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